Bore hole casings or liners are conventionally set in bore holes by a cementing process in which a cement slurry is forced down through the casing space and then upwardly around the outside of the casing to fill the annular space between the exterior casing surface and the surrounding wall of the formation. After solidification of the cement, communication between the casing and the producing zone is established by explosive perforation of the casing, e.g., by means of bullets or shaped charges which also penetrate the hardened cement to form passageways or ducts therethrough. This procedure is unsatisfactory as the bullets or charges tend to crack the cement around the passageways, thereby causing vertical communication, to wit, up and down movement around the casing from one perforation to another. This, in turn, prevents subsequent selective treatment through each perforation to the formation at the end of each duct or perforation, since injected treating material could travel up or down through cracked cement without permitting selective control at the injected places, i.e., the stratum of the formation at the end of each duct.
It will be appreciated that this prior art method of establishing communication between the producing formation and the interior of the casing is particularly disadvantageous in respect of completion of the well bore, be it by acidification, sand fracking, consolidation and the like. The reason for this is that treatment liquid which is forced down the casing and through the passages in the cement into the formation, of course, travels along a path of least resistance. Liquid thus enters the formation where least resistance is offered, while no liquid, or only minor amounts of liquid will penetrate formation strata which offer more resistance. Moreover, large amounts of liquid are wasted within the cracks and fissures within the cement wall. The injection of treatment liquid thus takes place in a most non-uniform manner, and no uniform flow rate of liquid through the various passages leading from the holes in the casing and through the cement into the formation is accomplished. This is particularly disadvantageous, when some or all of the passages are to be selectively blocked by sealing means, usually referred to in the art as ball sealers. Thus, in practice, when it is desired to block off communication between the formation and the passages, which lead from the formation through the cement into the casing interior, ball sealers are suspended in the treatment liquid, the ball sealers having substantially the same specific gravity as the treatment liquid. The intention is for the ball sealers to enter the passages and to block them. However, since the flow rate is non-uniform, it will be appreciated that the ball sealers, of course, have the tendency to enter only those passages through which liquid flows at a sufficient rate while no balls will be forced into the passages through which there is only a trickle or no flow of treatment liquid.
More recently, an improved method and device for establishing communication between the casing and the producing zone has been suggested. According to this suggestion, a plurality of duct-forming devices are welded or otherwise secured to the outside of the casing in alignment with holes machined into the casing wall. These duct-forming devices comprise telescoping tubes or sleeves which are in a retracted position during the positioning of the casing in the bore hole. When contact with a producing zone is to be made, these telescoping tubes are caused to project substantially horizontally toward the formation wall to make contact with the pay zone and to establish a permanent link between the pay zone and the casing. The cement slurry is introduced into the space between the casing and the formation wall immediately before the lateral telescoping of the tubes so that the cement sets around the tubes and the casing. The telescoping tubes of the duct-forming devices, as previously proposed, are made of steel or the like acid- and alkali-resistant metal and the outer, free tube end which ultimately contacts the producing formation is blocked by an acid and/or alkali soluble metal plug which is lodged within the tube in a sealing manner so as temporarily to prevent passage of material through the tube. When communication between the pay zone and the interior of the casing is to be established, an acidic or alkaline liquid is forced down the casing and into the laterally extending telescoping tubes to cause dissolution of the plug.
The present invention is directed to an improvement of metallic duct-forming devices of the kind referred to hereinabove, such duct-forming devices having been disclosed in a number of U.S. patents, for example, Nos. 2,775,304, 2,707,997, 2,855,049, 3,245,472, and 3,425,491, to which specific reference is had.
While the duct-forming devices referred to above constitute an important improvement in the art of well completion and recovery of formation fluids, the known duct-forming devices still do not permit effectively to establish a sufficient feeding rate for treatment liquid into the formation. Thus, when the acid-soluble plug has been dissolved, treatment liquid such as acid or sand-containing fracking liquid, which is forced down the casing will enter the formation through the duct-forming devices also along a path of least resistance. Further, when blocking of selected duct-forming devices by ball sealers is intended, the non-uniform feed rate previously referred to also applies to well completion apparatus in which duct-forming devices of the prior art are used.